The increased presence of ABS systems (antilock brake systems) on motor vehicles has led engineers to seek software methods for monitoring pneumatic tire pressure that use messages from the ABS controller because such methods can be highly cost effective due to the fact that each wheel is already equipped with a precision wheel speed sensor. The use of ABS wheel speed messages in methods for monitoring tire pressure are disclosed in various patents and other documents.
In July, 2001, the United States Department of Transportation published a document “Tire Pressure Monitoring System, FMVSS No. 138”, summarizing ITPM (indirect tire pressure monitoring) project progress. That document represents that ITPM's can report tire underinflation when the tire pressure difference is greater than 20-30% of their placard pressure. This is due to the fact that the effective radius of a pneumatic tire decreases very slightly when a tire is underinflated by about 15 psi (pounds per square inch).